This invention relates to the production of lignocellulosic fibres and formation of composite materials therefrom. It particularly relates to the production of such fibres and bonding with synthetic binders into composite materials.
There is considerable pressure on the world""s fibre resource. World-wide economic growth and development have created needs for converted forest products. While global fibre production systems are capable of meeting these overall demands there are some serious local and regional fibre shortages and resource management conflicts.
Many developing countries do not possess adequate forest reserves to cover their needs for fuelwood, industrial wood, sawn wood, and wood-based composition panels. However, many of these countries do have relatively large quantities of lignocellulosic materials available in the form of agricultural residues from annual crops. Annual plant fibres like cereal straw and the like are difficult to bond using conventional adhesives such as UF resins, PF-resins and PMDI binders.
The present invention is related, therefore, to a method of improving the bondability of lignocellulosic materials from annual plant fibres such as cereal straw by synthetic binders.
Composite materials such as particleboards, medium and high density fibreboards are mainly made from wood using binders such as acid curing amino-formaldehyde resins, alkaline curing phenol-formaldehyde resins, as well as polyisocyanate adhesives. Medium density fibreboards are fibreboards prepared using a dry technique as follows: Wood is subjected to thermomechanical pulping at a temperature of about 160 to 180xc2x0 C., then mixed with the resin and dried. Thereafter mats are formed from the fibres and pressed to form fibreboards. Particleboards, on the other hand, can be prepared from chips which are mixed with resins and the glued particles are spread to mats and pressed at high temperature to particleboards.
Recently, interest has been developed on using agricultural residues like wheat and rice straw and sunflower as a starting material for particleboards and medium density fibreboards. The main difficulty in using annual plant residues such as straw as a raw material for composites, is their poor bondability particularly using urea-formaldehyde resins. The reason for this is probably the specific morphological structure of the straw, where the waxy and silica layer encirculating the straw stem inhibit sufficient direct contact between the binder and the straw fibres. Other types of adhesives for example polymeric isocyanates have been tried. However, the mechanical strength as well as the water resistance of the boards made from straw and isocyanates are much lower than those made from wood using the same bonding conditions.
Therefore, it was the main aim of the invention to find a practical method to improve the bondability of annual plant residues towards bonding agents in general and particularly towards acid curing aminoplastic resins and also polyisocyanate binders.
While fibrous/particulate ligno/cellulosic materials have been treated by water/steam treatments with simultaneous or subsequent high shear treatment, use of lower temperatures has only been in the context of treatments for the manufacture of paper or similar materials and there has been no suggestion that this treatment when applied to lignocellulosic materials in the context of producing composites would enhance the fibrous or particulate material for forming into composite material. The process of the invention is also to be distinguished from producing composite materials from lignocellulosic materials in which there is an initial treatment at high temperature of at least 150xc2x0 C., usually 150xc2x0 C. to 170xc2x0 C. followed by defibration.
Thus many treatments have been described in the literature to improve the bondability of lignocellulosic materials in both particle and fibre form with synthetic resins. D. H. GARDNER and T. J. ELDER: (Bonding surface activated hardwood flakeboard with phenol-formaldehyde resin xe2x80x94Holzforschung 44(3): 201-206, 1990) added hydrogen peroxide, nitric acid or sodium hydroxide to enhance bonding characteristics of flakes using phenol-formaldehyde resins as a binder. Dimensional stability and internal bond strength were significantly reduced and it was shown that the chemicals did not change the wood surface, but rather they reacted with the resin.
J. McLAUGHLAN and C. R. ANDERSEN: (In-Line fibre pretreatments for dry process medium density fibreboard: Initial Investigationsxe2x80x94Paper presented at the Symposium Pacific Rim Bio-Based Composites, Rotorua, New Zealand 9-13 Nov. 1992, Symposium Proceedings, page 91-99, 1992) tried many treatments to enhance the bondability of fibres towards bonding with urea-formaldehyde resins for the production of medium density fibreboards. The treatments include exposure to wet and dry heat, compression with heat and heat in combination with chemicals. The chemicals include 1% and 10% addition of aluminum sulphate, which is used in the hard board manufacture to control the pH value of the stock and 1% and 10% chromium trioxide. Almost all the treatments resulted in boards with reduced properties compared to the control.
SIMON AND L. PAZNER): (Activated self-bonding of wood and agricultural residuesxe2x80x94Holzforschung 48: 82-90, 1994) investigated the influence of the hemicellulose content of the self-bonding behaviour of different raw materials including annual plants and concluded that there is a straightforward relation between the hemicellulose content in the raw materials and the bonding strength of composites prepared therefrom. According to this work hemicelluloses do have adhesive properties, however, bonds created using hemicellulose adhesives have almost no wet strength.
In a recent publication LIAN ZHENGTIAN and HAO BINGYE: (Technology of rice-straw particleboards bonded by Urea-formaldehyde resin modified by isocyanatexe2x80x94Paper presented at the Symposium Pacific Rim Bio-Based Composites, Rotorua, New Zealand 9-13 Nov. 1992 Symposium Proceedings, page 295-301, 1992) mentioned that slight improvement of bondability of straw can be achieved by destroying the waxy layers encircling the stem of straw, however, the bondability was still very poor and the boards made still could not meet the requirements of common standards.
In DE-A-36 09 506 is described a modified standard dry process for the production of MDF in which UF resin is injected after treatment of wood particles with overheated steam and separation of steam from the treated fibres. The treatment of the fibres is by a conventional disc refiner.
In US-A-3 843 431 composite panels are produced from fibres prepared using as starting material scraps, shavings, sawdust. The raw material is blended with water and ground with the use of a double-disc attrition mill.
In WO-A-93 25358 MDF is produced according to the standard dry process involving wood chips pre-treated prior to defibration. The pre-treatment procedure involves the impregnation of raw material with Na2SO3/NaHSO3, and heating at a temperature between 150-200xc2x0 C.
The aim of the present invention to develop a method for the treatment of annual plant fibres, so that their bondability to synthetic resins is significantly improved and the production of composite panels with properties that meet the requirements of common standards is effected.
It has been discovered that thermal treatment of straw or other annual plant fibres with water or steam at temperatures between 40-120xc2x0 C., and preferably between 60 -100xc2x0 C. accompanied by or followed by defibration of the fibres using high shear forces destroys the morphological structure of the straw and increases tremendously its affinity towards bonding.
According to the invention, therefore, there is provided a method for producing composite materials wherein a lignocellulosic material which is an annual plant fibre residue is subjected to treatment with water or steam at 40xc2x0 to 120xc2x0 C. and simultaneously or subsequently is subjected to a high shear treatment and thereafter is formed into a composite material. The invention also relates to a lignocellulosic material which is an annual plant fibre residue which has been subjected to such water/steam treatment and high shear treatment and is in a form suitable for bonding into a composite. The invention also relates to a composite material in which at least part of the fibre content is derived from said treated annual plant fibre residue.
Defibration in the sense of this invention means disruption of the morphological structure of straw leading to the creation of individual fibres. The treatment destroys the waxy and silica layer of straw, leading to higher accessibility of individual fibres to the binder.
Lignocellulosic annual plant fibre residues which can be used in this invention are to be distinguished from wood products or other plant products which do not grow on an annual basis. They include rice straw, rice husks, wheat straw, rye straw, cotton stalks, miscanthus, sorghum and sunflower.
Binders or bonding agents are those conventionally employed in forming composite products and include both acidic and alkaline type binders. Typical bonding agents are amino resins, phenolic resins, resorcinol resins, tannin resins, isocyanate adhesives or mixtures thereof. Thus resins which can be used to bond treated straw fibres include urea-formaldehyde resins (UF-resins), melamine-urea-formaldehyde resins (MUF-resins), melamine resins (MF-resins), phenol-formaldehyde resins (PF-resins), resorcinol-formaldehyde resins (RF-resins), tannin-formaldehyde resins (TF-resins), polymeric isocyanate binders (PMDI) and mixtures thereof. The resins can be added in the amount of 5-15% based on dry straw materials employed in the final composite.
The hydro-thermal treatment can be with water alone or with water and treating agents as will be later described.
High shear treatment is an application to the fibre of interaction between mechanical surfaces which imposes a high shear force on the fibre as distinct from prior art low shear grinding or similar attrition treatments. Those skilled in the art are well aware of high shear devices which are exemplified by twin screw extruders, disc refiners, ultra turrax or any other suitable high shear mill. The rate of extrusion depends upon the conditions used and also the type of machine applied and can differ from 5 kg/h to 20 t/h.
The intensity of shearing applied must be such that, depending upon the type of composite which is to be prepared from the straw, one achieves a substantial defibration of the straw. For MDF and high density fibreboard, it is necessary to reach more or less complete defibration of the straw, so as to produce treated straw that displays sufficient bonding affinity towards a UF resin, to enable formation of boards having certain desired properties. Medium density fibreboards cover a wide range of densities between 0.6 and 0.8 g/cm3 depending on their thickness and field of application. Boards with density lower than 0.5b g/cm3 are not common, but can be produced. The quality required depends on the field of application of the board and its thickness:
For particleboards on the other hand, partial difibration would be sufficient. Particleboards are prepared in the density range of 0.4 to 0.85 g/cm3 depending upon their field of application and thickness. Boards with density lower than 0.5 g/cm3 are low-density boards, between 0.5 and 0.7 g/cm3 are medium density and greater than 0.7 g/cm3 are high density boards. Also, in the case of particleboards, the requirements depend on the field of application and thickness of the boards.
The properties of the boards made from straw can be further improved if the straw is treated with various chemicals which are fibrous property lignocellulose modification agents. These reagents can be used either alone or in combination and include metal hydroxides, such as lithium, sodium, potassium, magnesium and aluminium hydroxide, organic and inorganic acids, such as phosphoric, hydrochloric, sulphuric, formic and acetic acid, salts, such as sodium sulphate, sodium sulphite and sodium tetraborate, oxides, such as aluminum oxide; various amines and urea, ammonia, as well as ammonium salts. These reagents can be used in the form of water solution or suspension in quantities of from 0.01 to 10% based on dry material.
The chemical treatment and the defibration are carried out in one step, by subjecting the straw to a stream of water during the high shear stage, containing the amount of chemical needed to upgrade the properties of the amino resin bonded boards. After the defibration, the fibres produced can be dried using conventional dryers used in particleboard factories, e.g. a drum dryer or a tube dryer, like that used in medium density fibreboard mills. From then onwards, the dried fibres follow the conventional procedures as for the production of particleboard or medium density fibreboard.
It is also one of the embodiments of this invention to mix the annual plant fibres with a binder or a binder mixture already in the high shear machine. UF, MUF, MF, PF, RF and TF resins can be employed for this purpose. In the case of amino resins, the adhesive can be added in a pre-catalysed or latently catalysed or non-catalysed state. A catalyst can also be added separately in the high shearing stage. Mixtures of resins like UF-polyisocyanates can also be used in the same way.
The addition of a sizing agent is not obligatory. However, it can be added, if appropriate, either in the high shear machine or separately. Other components of a standard glue mixture like formaldehyde scavengers and extenders can also be added in the same way.
The final composite materials can be panel products, reconstituted lumber products and moulded articles including particleboard, waferboard and fibreboard.
The resulting composition boards produced from treated straw fibres are very different from the boards produced using standard chopped straw. The appearance, surface smoothness and core density profile are superior, approaching the quality of medium density fibreboards. Excellent edge properties and improved board machineability are further advantages of the process. High density boards can be produced, without the need to apply high board forming pressures.
In a further embodiment of the invention, treated straw fibres can be used as a partial substitute for wood chips in the production of wood particleboards. The benefit is an improvement of board general appearance, density profile and machineability. Wood substitution levels of between 1-50%, and preferably between 10-30% can be employed. The conventional procedure for the production of particleboards is applied.